This invention relates to an optical information recording medium such as a recordable compact disc (namely, CD-R) or a recordable digital video disc (namely, DVD-R).
An optical information recording medium on which information can be only once recorded by means of a laser beam (i.e., an optical disc of write-once type) is known as a recordable compact disc (CD-R), and is widely used in practice. The optical disc of CD-R type generally has a multi-layered structure typically comprising a disc-shaped transparent substrate (support), a recording layer comprising an organic dye (often referred to as xe2x80x9crecording dye layerxe2x80x9d), a light-reflecting layer comprising a metal (hereinafter, often referred to as simply xe2x80x9creflecting layerxe2x80x9d), and optionally a protective layer made of resin overlaid in order.
Large recording capacity is often needed in recent days, but an optical disc of CD-R type does not always have fully satisfactory large information-recording capacity. Therefore, an optical disc having satisfactory large information-recording capacity has been desired and proposed. For example, a recordable DVD (i.e., recordable digital video disc, DVD-R), which information can be recorded on and read out from by means of a laser beam having a shorter wavelength than that for CD-R, has been proposed. The optical disc of DVD-R type is formed by laminating two composites having a layered structure.
Each of the layered composites comprises a transparent disc substrate provided with a guide groove (i.e., pregroove) for tracking of the laser beam, a recording layer comprising an organic dye, a light-reflecting layer, and a protective layer arranged in order. The layered composites are combined with an adhesive so that the recording layers would be placed inside. The pregroove on the substrate of DVD-R generally has a width of 0.74 to 0.8 xcexcm, which is less than a half of the width of the pregroove on a substrate of CD-R. Otherwise, the optical disc of DVD-R type is formed by combining a protective disc plate and the layered composites comprising a substrate, a recording layer, a light-reflecting layer and a protective layer arranged in order.
For writing (i.e., recording) and reading (i.e., reproducing) the information, a CD-R is irradiated with a laser beam having a wavelength in the range of 770 to 790, typically approx. 780 nm, while a DVD-R is irradiated with a laser beam of visible wavelength region (generally having a wavelength of 600 to 700 nm). By the irradiation of the laser beam, irradiated area of the recording dye layer is locally heated to change its physical or chemical characteristics, and pits are formed in the irradiated area of the recording layer. Since the optical characteristics of the formed pits are different from those of the area having been not irradiated, the digital information is optically recorded. The recorded information can be read by a reproducing procedure generally comprising the steps of irradiating the recording layer with the laser beam having the same wavelength as that employed in the recording procedure, and detecting the light-reflection difference between the pits and their periphery.
Japanese Patent Provisional Publication No. 2-78038 describes an optical information recording disc of CD-R type whose substrate has a pregroove having a radiused bottom corner and a radiused top corner on its both sides. It is described that an optical disc having the substrate shows a high reflectance and ensures good tracking performance. The pregroove having the radiused corners is formed by coating an organic solvent on a molded substrate having a pregroove, so that the corners of the pregroove can be slightly dissolved in the solvent to give the radiused corners. Therefore, the top corners of the pregroove illustrated in the above publication have an equal radius on the substrate from its inner side to outer side.
It is known that a molded resinous substrate has warp particularly on its outer periphery. An optical recording disc using such warped substrate shows side- runout when the optical disc is rotated for recording an information or reading out the recorded information. Accordingly, a substrate has been so produced that the warp would be produced as small as possible. However, since an increased rotation rate of an optical disc for recording or reading out an increased amount of information within a short period has been adopted, the warp of the substrate is apt to cause trouble even if the warp is not large. Particularly, if the optical disc is rotated at a rate as much as twice or more, particularly four times or more of the standard rate, the side-runout on the outer periphery of the disc causes troubles such as increase of jitter values, resulting in production of error in the information read-out procedure. The trouble is supposed in one aspect to be caused that the side-runout of the disc in the recording procedure causes deformation of shapes of pits for the information recording. The substrate having a substrate in which the pregroove has the radiused top corners is relatively free from the deformation of pit shapes in the information recording procedure. However, it is desired to provide an optical information recording disc which is favorably employable in the information recording and reproducing procedures which are performed at an increased disc rotation rate.
Accordingly, it is an object of the present inventors to provide an optical information recording disc such as CD-R or DVD-R which is favorably employable in the information recording and reproducing procedures which are performed at an increased disc rotation rate.
It has been now discovered by the inventors that an optical information recording disc having a substrate on which a pregroove has a radiused (or rounded) top corner whose radius is more generous on the outer periphery of the disc than on the middle area of the disc is advantageous for obviating the errors produced on the information recording procedure and the information reproducing procedure. This is supposed that the formation of pits in the recording layer in the pregroove having a highly generous corner as well as read-out of the recorded information from the layer in the pregroove having a highly generous corner is less effected by the side-runout of the peripheral area of the recording disc.
Accordingly, the present invention resides in an optical information recording disc comprising a transparent disc substrate provided with a spiral pregroove, a recording dye layer placed in the pregroove on which information is recorded by irradiation with a laser beam, and a light-reflecting layer, arranged in order, wherein the pregroove formed in the area between an inner circle having a radius corresponding to a half of a radius of the disc substrate and an outer circle having a radius corresponding to 19/20 of the radius of the disc substrate has a radisued top corner at least one side, under the condition that the radius of the radiused corner on the outer circle is more generous than the radius of the radiused corner on the inner circle under the following conditions:
W190xe2x88x92W150 less than W290xe2x88x92W250
wherein W190 means a width of the pregroove on the inner circle which is measured at a depth of 10% of the depth of the pregroove and W190 means a half width of pregroove on the inner circle, while W290 means a width of the pregroove on the outer circle which is measured at a depth of 10% of the depth of the pregroove and W250 means a half width of the pregroove on the outer circle.
If there is formed no spiral pregroove on the outer circle, an outermost spiral pregroove (which is generally formed at 1 mm from the outer end of the substrate disc) is adopted in place of the pregroove on the outer circle.
The optical information recording disc of the invention is generally utilized as a recordable compact disc (CD-R) or a recordable digital video disc (DVD-R).
In the optical information recording disc of the invention, it is preferred that the radius of the radiused corner of the pregroove continuously increases from the pregroove on the inner circle to the pregroove on the outer circle.
Both of the pregroove on the inner circle and the pregroove on the outer circle preferably have a radiused corner on their both sides.
The CD-R of the invention is favorably employed in a method of recording information which comprises irradiating the CD-R with a laser beam having a wavelength of 770 to 790 nm.
The DVD-R of the invention is favorably employed in a method of recording information which comprises irradiating the DVD-R with a laser beam having a wavelength of 600 to 700 nm.